264 LECTURE XXIX. 



water is applied, and in this case the velocity must be about 500 feet 

 in a second. 



Bellows may be used for the ventilation of a mine, either by forcing 

 air into it, or by drawing it out through a pipe connected with the valve. 

 The wind may also be received by the mouth of a tube a little conical, and 

 may be made to cause a current where it is conveyed ; such an instrument 

 is sometimes called a windsail, or a horse head. It has been proposed to 

 draw the air up through a pipe by the lateral friction of a current of air 

 received by such a funnel, but the effect would probably be too small to be 

 of much practical utility. 



A corn fan is turned by the hand or by machinery ; its simplest opera- 

 tion is to cause a portion of air to revolve with it, and to create a wind in 

 the direction of its circumference. But when a small fan is made to revolve 

 with great rapidity, as in Papin's Hessian bellows, the centrifugal force 

 causes the air admitted at the centre to rush towards the circumference, 

 and to pass with great velocity through a pipe inserted there. The com- 

 mon ventilator placed in windows, which revolves in the same manner as 

 a smoke jack, in consequence of the impulse of a current of air, serves 

 only to retard a little the entrance of that current, to disperse it in some 

 measure in different directions, and to prevent any sudden increase of the 

 intensity of the draught ; but it has little or no power of acting on the air, 

 so as to prevent the decrease of the velocity of the current. (Plate XXIV. 

 Fig. 384.) 



The operation of heat affords us also a very effectual mode of ventilation. 

 Its action upon air at common temperatures occasions an expansion of 

 about T ^ ly for every degree that Fahrenheit's thermometer is raised ; the air 

 becomes in the same proportion lighter, and the fluid below it is conse- 

 quently relieved from a part of its weight : the pressure of the surrounding 

 atmosphere, .therefore, preponderates, and the lighter column is forced 

 upwards. When the shaft of a mine communicates with the external air 

 at two different heights, there is generally a sufficient ventilation from the 

 difference of the temperatures of the air in the shaft, and of the surrounding 

 atmosphere: for the temperature of the earth is nearly invariable, it 

 therefore causes the air in the shaft to be warmer in winter than the 

 external air, and colder in summer ; so that there is a current upwards in 

 winter, and downwards in summer ; and in the more temperate seasons, 

 the alternations take place in the course of the day and night. For a 

 similar reason there is often a current down a common chimney in sum- 

 mer ; but when the fire is burning, the whole air of the chimney is heated, 

 and ascends the more rapidly as the height is greater. It would be easy, 

 from the principles of hydraulics, if the length of the chimney, and the 

 mean temperature of the air in it were given, to calculate the velocity of 

 the draught : thus if the height of the chimney were 50 feet, and the air 

 contained in it 10 degrees hotter than the external air, the expansion would 

 be one fiftieth, and the pressure of the whole column being diminished one 

 fiftieth, the difference would be equivalent to a column of one foot ifc 

 height, and such a column would represent the pressure causing the 

 draught, which might, therefore, be expected to have a velocity of feet 



